1. Field of the Invention
The invention relates to an electromagnetic ultrasound transducer, particularly for receiving linearly polarized shear waves, what are known as SH ultrasound waves, from an electrically conductive workpiece, with a magnetizing unit, which provides a side facing the workpiece, along which a number n of permanent magnets is in each case attached in at least two rows arranged indirectly or directly next to one another in such a manner that the magnetic polarities which face the side and can be assigned to the permanent magnets alternating along a row periodically with a period length which corresponds to a trace wavelength λs and also with a HF coil arrangement with conductor sections which in each case can be assigned along the at least two rows and run parallel to one another, through which current can pass in mutually opposite directions.
2. Description of the Prior Art
Electromagnetic ultrasound transducers are used for coupling ultrasound waves into or out of workpieces without a coupling means, for example for non-destructive thickness measurement or material investigation, in order to detect material inhomogeneities in the form of cracks or material flaws.
The excitation and also reception principle on which the electromagnetic ultrasound transducers are based, is based on the interaction between an electromagnetic high-frequency field prevailing close to the surface within the workpiece and a static or quasi-static magnetic field laid over the same. With the aid of an electric coil with a predetermined geometry and number of turns arranged close to the surface on the workpiece, which coil is loaded with a HF current pulse/burst signal, eddy currents are induced within what is known as the skin depth of the electrically conductive workpiece, close to the workpiece surface. The two-dimensional distribution of eddy currents is mirror-inverted to the geometry of the electric coil arrangement. If the eddy currents forming within the skin depth of the workpiece are overlaid with a static or quasi-static magnetic field parallel or perpendicular to the material surface, then spatial and temporally periodic elastic material displacements result due to Lorentz forces acting within the workpiece, which are the cause of the damping of ultrasound waves within the workpiece.
The detection or the reception of ultrasound waves takes place in a reciprocal manner. Then an elastic wave propagating within the workpiece close to the surface in the presence of a magnetic field prevailing in this workpiece region generates an electrical field proportional to the displacement speed of the elastic wave, which induces a proportional electrical voltage into an electrical coil resting on the workpiece surface by inductive coupling with the same. The electrical voltage is used as detection signal for the ultrasound wave within the workpiece. The voltage signal levels arising in this process typically lie in the range of a few μV, so there is a requirement for a strong and low-noise pre-amplification of the electrical voltage signals for a reliable signal evaluation and assessment, which additionally are to be subjected to an electrical filtering which is as narrow-banded as possible, in order to generate ultrasound wave signals that can be evaluated.
Typically, the impedance of the electrical coil of an EMUS transducer, which is particularly suitable for the reception of ultrasound waves, is of high-ohmic configuration, in order to generate the greatest possible level of the induced voltage signals from the ultrasound signals. However, due to its electrical inductance, the electrical coil used is also suitable to receive other electromagnetic signals which originate from externally inductively acting electromagnetic signal sources and as such influence the reception and detection of ultrasound waves in an interfering manner. All reception signals inductively converted by the electrical coil into electrical voltage signals, that is both useful and interference signals, pass through the same amplification and filtering chain, so that a differentiation between interference and useful signals is not readily possible.
An ultrasound probe based on the previously described principle of coupling of ultrasound waves into or out of a workpiece without a coupling means is disclosed in DE 42 23 470 C2. The probe generates linearly polarized transverse waves which are both horizontally and vertically polarized. Here, a permanent magnet arrangement is used which produces an inhomogeneous magnetic field in the region of a workpiece close to the surface with a direction in space orientated perpendicularly to the workpiece surface. The permanent magnet arrangement has individual permanent magnet strips lying next to one another with periodically alternating magnetic polarities facing the workpiece surface in each case.
A further arrangement for introduction of sound and detection of ultrasound waves in ferromagnetic workpieces, as for example in pipelines, without a coupling means, is disclosed in DE 195 43 481 C2. In order to be able to damp horizontally polarized transverse waves within a workpiece to be tested with a spatially predetermined directional characteristic, an embodiment illustrated in FIG. 3 of an ultrasound transducer provides a permanent magnet arrangement with a multiplicity of individual permanent magnets arranged in rows which in each case are identically configured in terms of shape and size with the magnetic polarities of periodically alternating along a row. In order to obtain a spatially directed radiation characteristic, the individual permanent magnets in a row are arranged mutually offset to those in the directly adjacent row by half the width of an individual permanent magnet. This corresponds to a quarter of what is known as the trace wavelength λs. In addition, conductor sections of two HF coil arrangements are attached along the individual rows of individual permanent magnets through which current flows in opposite directions. Further details are available from the previously mentioned published document.
Particularly suitable for the reception of SH ultrasound waves from an electrically conductive workpiece are prior art EMUS transducers, of which two embodiments are schematically illustrated in the FIGS. 2a and b, which in each case show the side of the magnetic arrangement M and the HF coil arrangement HF facing the workpiece.
In the prior art in FIG. 2a, permanent magnets 1 are arranged along two rows R1 and R2 in such a manner that the magnetic polarities facing the workpiece periodically alternate in sequence along the rows R1 and R2 N is for magnetic north and S is for magnetic south. A magnet arrangement M illustrated in FIG. 2a therefore impresses an non-homogeneous static magnetic field with a trace wavelength λs into a workpiece, which is determined by the period length, that is the extent of two permanent magnets along a row.
Further, a HF coil arrangement HF is arranged on the side illustrated in FIG. 2a of the magnetizing unit M facing the workpiece. During a test use in each case, conductor sections L1, L2 run along the at least two rows R1 and R2 parallel to one another through which current can pass in mutually opposite directions (see current arrows).
In an analogous development to the prior art illustrated in FIG. 2a, an embodiment illustrated in FIG. 2b provides the arrangement of permanent magnets 1 in four divisible and mutually adjacent rows R1 to R4. In this case also, the HF coil arrangement HF is constructed in such a manner that current can pass through the conductor sections L1 to L4 which in each case run along the rows R1 to R4 in mutually opposite directions. The HF coil arrangement is in this case divided into two part coils T1 and T2 which are connected to one another.
In order to counteract the prior art problem of the simultaneous reception of ultrasound signals and interference signals and to obtain an improved signal to noise ratio, the use of a differential amplifier for signal amplification is the basis of the variant illustrated in FIG. 2c. In this context, an attempt has been made to separate the two left and right part coils T1, T2 illustrated in FIG. 2b and combine them with a differential amplifier. A design of this type is disclosed in FIG. 2c. The connections E1 and E2 of the part coils are connected to the respectively inverting and non-inverting inputs of a differential amplifier D. The two other connections of the part coils T1 and T2 are connected to ground in the illustrated example. This approach does not achieve the desired goal of an effective noise or interference suppression. Although the voltage signals which originate from received ultrasound pulses have a relative phase of 180° at the corresponding poling of the coils T1 and T2 for reception specified in FIG. 2c, this is also true for the interference signals which have an identical phase shift of 180°. The only advantage of the configuration variant illustrated in FIG. 2c lies in the fact that the voltage amplitude is virtually doubled by the addition of the two reception signals within the framework of the differential amplifier. As a result, further signal evaluation can be improved by the digitizing of the signal levels. Nonetheless, the signal levels of the interference signals are amplified in the same manner, so no improvement of the signal-noise ratios can be achieved.